The background of the present invention will be explained using a 3GPP wireless communication system shown in FIG. 1 as an example.
FIG. 1A is a block diagram showing a general wireless mobile communication system and FIG. 1B is a format diagram showing an example of a frame structure of a signal transmitted/received between a base station and a mobile terminal, referred to as “user equipment”. As shown in FIG. 1B, a frame (subframe) has a frame length of, for instance, 0.5 ms, and while a plurality of blocks having a predetermined length are time-multiplexed, a plurality of resource blocks are frequency-multiplexed in a predetermined frequency band with a predetermined number N of sub-carriers as a resource block.
In wireless communication, a reference signal is generally multiplexed with a transmission signal, and using the received reference signal, channel estimation, link adaptation, and CQI (Channel Quality Indicator) measurement for scheduling are performed.
In order to perform channel estimation using the reference signal, the receiver must know the sequence of the reference signal transmitted. As such a sequence, CAZAC (Constant Amplitude Zero Auto-Correlation) sequence has gained much attention in recent years. Since CAZAC sequences have a constant amplitude in the time domain, PAPR (Peak-to-Average Power Ratio) can be minimized. Further, they performs well in channel estimation in the frequency domain because their amplitude is also constant in the frequency domain. Further, since the CAZAC sequences have perfect autocorrelation properties, they are suitable for the timing detection of the received signal. They have also gained much attention as sequences suitable for single carrier transmission. Therefore, Zadoff-Chu sequences, a kind of the CAZAC sequences, are used in 3GPP LTE (Long Term Evolution) as uplink reference signal sequences (Non-Patent Document 1). The Zadoff-Chu sequence Pk is represented by the following equations (1) or (2).
[EQUATION 1]
When a sequence length L is an even number:Pk=(ak(0), ak(I), . . . , ak(L−1)), ak(n)=exp(−j2πkn2/2/L)  (1)                (n=0, 1, . . . , L−1)[EQUATION 2]        
When the sequence length L is an odd number:Pk=(ak(0), ak(1), . . . , ak(L−1)), ak(n)=exp(−j2πkn(n+1)/2/L)  (2)                (n=0, 1, . . . , L−1)        
However, as described below, the number of available CAZAC sequences is limited. The sequence number of CAZAC sequences depends on the sequence length L, and in the case of the Zadoff-Chu sequences, the sequence number is the greatest when the sequence length L is a prime number. The greatest sequence number at this time is equal to (L−1). When it is necessary to generate as many unique CAZAC sequences P1, P2, P3 . . . as possible and assign each of them to a base station, it is optimal to use sequences whose sequence length L is a prime number in order to have the greatest number of sequences. As described below, a reference signal has conventionally been generated using this method.
FIG. 2 is a flowchart showing a conventional method for generating a reference signal. First, a CAZAC sequence having a sequence length of a prime number closest to the sub-carrier number N in the resource block is selected (step S10). Then, a plurality of reference signal sequences are generated from the selected CAZAC sequence by the well-known cyclic shift method (step S11). The generated reference signal sequences can be assigned to a plurality of user equipments UE (mobile terminals).
However, the sub-carrier number N, the resource size of a reference signal, is not normally a prime number. As shown in FIG. 1B, in LTE, the resource size of a data signal is 12 sub-carriers (1 resource block) multiplied by an integer, and the number of the sub-carriers in a resource block of the reference signal corresponding to this is also 12 sub-carrier multiplied by an integer. Therefore, the sub-carrier number N in a resource block of the reference signal will be 12, 24, 36, . . . . Meanwhile, a sequence length of a prime number closest to the sub-carrier number N has to be used in order to have a greatest number of sequences. These are 11, or 13, 23, 37 . . . , and they don't coincide with the sub-carrier number N in a resource block of the reference signal. For such a case where the resource size (the sub-carrier number) of the reference signal and a length of a prime number don't coincide, several methods in which CAZAC sequences having a length of a prime number are allocated to sub-carriers in the resource block of the reference signal have been proposed.
FIG. 3A is a diagram showing the frequency structure of the resource block of the reference signal; FIG. 3B is a sequence allocation diagram when a sequence having a length of a prime number is allocated to sub-carriers without any change; FIG. 3C is a sequence allocation diagram when the sequence is allocated with one code cyclically copied; and FIG. 3D is a sequence allocation diagram when a sequence is allocated with one code being truncated.
The method indicated by FIG. 3B is described in Non-Patent Document 2. If a sequence (#1 to #11) with the number of its length smaller than the sub-carrier number N in the resource block is allocated without any change, no reference signal will be allocated to the last of 12 sub-carriers.
In the method shown in FIG. 3C, in which one code is cyclically copied, by compensating the difference between the sequence (#1 to #11) and the sub-carrier number N=12 in the resource block by performing a cyclic copy, the sequence is adjusted so that it matches the sub-carrier number N=12 in the resource block (Patent Document 1).
Further, in the method shown in FIG. 3D, in which one code is truncated, by truncating the sequence (#1 to #13) with the number of its length larger than the sub-carrier number N=12 in the resource block at the sub-carrier number N=12 in the resource block, the sequence is adjusted so that it matches the sub-carrier number N=12 in the resource block (Patent Document 1).
[Non-Patent Document 1]
3GPP TR 25.814 v2.0.0, June, 2006
[Non-Patent Document 2]
3GPP R1-063369 Nokia, “CAZAC Sequence Length for E-UTRA UL,” Nov. 6-10, 2006
[Patent Document 1]
US2005/0226140A1